1. Field of the Invention
The present invention relates to a belt drive control device which performs drive control of a belt stretched over a plurality of supporting rotary members, a belt device using the belt drive control device, an image forming apparatus, such as a copier, a printer, a facsimile machine, and a digital multifunctional machine having the functions of these apparatuses, which uses the belt device, and a belt drive control method for the belt drive control drive.
2. Discussion of the Background Art
As an example of an apparatus using a belt, an image forming apparatus using a belt, such as a photoconductor belt, an intermediate transfer belt, and a sheet conveying belt, is known. In such an image forming apparatus, highly accurate drive control of the belt is necessary to obtain a high-quality image.
Next, a description is given of an example of a related-art tandem-type electrophotographic image forming apparatus using an intermediate transfer method will now be described.
The related-art image forming apparatus includes an intermediate transfer belt, image forming units, a laser exposure unit, a fixing device, photoconductor drums, and so forth. In the image forming apparatus, the image forming units are sequentially arranged in the conveying direction of a recording sheet (i.e., a recording medium) to form single-color images of yellow (Y), magenta (M), cyan (C), and black (K), for example. The laser exposure unit forms electrostatic latent images on the surfaces of the photoconductor drums. The electrostatic latent images are then developed in the image forming units to form toner images (i.e., visible images). Then, the respective single-color images formed on the surfaces of the photoconductor drums in the image forming units are transferred onto the intermediate transfer belt such that the images are superimposed on one another. Thereafter, toner in the toner images is fusion-pressed and fixed onto the recording sheet by the fixing device to form a color image on the recording sheet.
In this type of image forming apparatus, a phenomenon known as color shift occurs if the moving speed of the recording sheet, i.e., the moving speed of the intermediate transfer belt, is not kept constant. The color shift is caused by a relative shift in the transfer position of the single-color images superimposed on the recording sheet. The color shift results in, for example, the blurring of a fine line image formed by superimposed images of a plurality of colors, and the formation of white spots around the outline of a black text image appearing in a background image formed by superimposed images of a plurality of colors.
Further, in the above-described related-art tandem-type image forming apparatus, and also in an image forming apparatus which uses a belt as a recording medium conveying member for conveying a recording medium or as an image carrying member for carrying an image to be transferred onto the recording medium, such as a photoconductor and an intermediate transfer member, banding occurs if the moving speed of the belt is not kept constant. Here, banding refers to unevenness in image density caused by variation in the belt moving speed in an image transfer process.
That is, an image portion transferred at a relatively high belt moving speed has a shape expanded in the circumferential direction of the belt, as compared with the original shape of the image portion. Conversely, an image portion transferred at a relatively low belt moving speed has a shape contracted in the circumferential direction of the belt, as compared with the original shape of the image portion. Accordingly, the density is reduced in the expanded image portion and increased in the contracted image portion.
As a result, the image density becomes uneven in the circumferential direction of the belt, and the banding occurs. The banding is clearly perceivable by the human eye, if the formed image is of a single pale color.
As described above, it is necessary for preventing the color shift and the banding to perform highly accurate drive control of a circular belt, such as a photoconductor belt, an intermediate transfer belt, and a conveying belt, for moving the belt at a constant moving speed. To provide such highly accurate drive control of a belt, a background drive control method controls the rotation of a drive roller to maintain a constant rotational speed of the drive roller which drives the belt. According to the drive control method, the rotational angular velocity of a motor which serves as a drive source and the rotational angular velocity of a gear which transmits torque generated by the motor to the drive roller are kept constant to maintain a constant rotational speed of the drive roller.
According to the above-described belt drive control method, however, if the thickness of the belt varies particularly in the moving direction of the belt, it is difficult to maintain a constant moving speed of the belt, even if the rotational angular velocity of the drive roller is kept constant.
Known image forming apparatuses addressing the above-described issue include, for example, the five background image forming apparatuses described below.
A first background image forming apparatus controls the rotation of the drive roller as follows: In order to highly accurately extract the amplitude and the phase of an alternating current component corresponding to a variation in thickness of the belt in the circumferential direction thereof using a less expensive arithmetic processing device than a device for performing the Fourier transform, an encoder rotation detection unit detects the angular displacement or the angular velocity of a driven roller. Then, on the basis of the detection result, a belt cycle variation detection unit performs quadrature detection to extract the amplitude and the phase of a belt alternating current component of an angular displacement or an angular velocity having a frequency corresponding to the variation in thickness of the belt. On the basis of the thus-extracted amplitude and phase, a target function calculation unit generates a target function. Further, on the basis of the target function, a target reference signal generation unit generates a target reference signal. Then, a comparator compares the target reference signal with an FB signal representing the detection result of the encoder rotation detection unit. On the basis of the comparison result, a motor is controlled. Thereby, the rotation of the drive roller is controlled.
According to a second background image forming apparatus, a drive signal output by a motor and input to a conversion unit is converted into the rotational angular velocity of a driven roller to enable an image forming operation to be performed even during the extraction of the amplitude and the phase of a belt alternating current component of a rotational angular displacement or a rotational angular velocity having a frequency corresponding to a periodical variation in thickness of the belt in the circumferential direction thereof. Then, a comparator compares an output drive signal with the input drive signal converted by the conversion unit to obtain a variation component attributed to a belt thickness variation in one belt cycle. A cycle variation sampling unit then stores in a memory the variation component attributed to the belt thickness variation in one belt cycle. Then, on the basis of the variation component for one belt cycle stored in the memory, a variation amplitude and phase detection unit detects the amplitude and the phase of the belt cycle variation component.
To prevent irregular rotation of a rotary member, a third background image forming apparatus, which is a color image forming apparatus, includes a drive device, a first speed detection device, a second speed detection device, a Fourier transform device, a correction data calculation device, a correction data storage device, and a drive control device. The drive device drives to rotate the rotary member. The first speed detection device outputs a signal having a frequency proportional to the rotational speed of the drive device. The second speed detection device outputs a signal having a frequency proportional to the rotational speed of the rotary member. The Fourier transform device performs Fourier transform processing on the signal output by the second speed detection device. The correction data calculation device extracts a specific frequency component to be corrected, and calculates and generates correction data from the frequency and the amplitude of the frequency component. The correction data storage device stores the correction data calculated by the correction data calculation device. The drive control device controls the rotational speed of the drive device on the basis of the respective detection signals output by the first and second speed detection devices and the correction data read from the correction data storage device.
A fourth background image forming apparatus, which is a color image forming apparatus, is designed to reduce a positional shift and expansion and contraction of an image in a transfer operation, without substantially increasing the mechanical accuracy of the apparatus. The color image forming apparatus includes a transfer member used to transfer a toner image formed on a photoconductor onto a recording sheet, and a drive shaft used to rotate the transfer member. A storage device previously stores the information of a change in the angular velocity of the drive shaft occurring when a drive motor for driving the transfer member is rotated at a constant angular velocity. Then, the transfer operation is performed while the information of the change in the angular velocity is read from the storage device and the angular velocity of the drive motor is changed on the basis of the information.
A fifth background image forming apparatus prevents the color shift in the transferred image attributable to the variation in the moving speed of the intermediate transfer belt or the like, due to such factors as an unclear mark and a scratch on the belt. The image forming apparatus detects the moving speed of the circular belt, which carries a recording sheet or a toner image and is provided with marks at predetermined intervals, and controls the moving speed to be constant to control the shift in the transfer position of the toner image transferred onto the recording sheet or the circular belt. The image forming apparatus includes a device which detects the moving speed of a mark, a device which calculates and acquires, on the basis of the moving speed, the extend of adjustment, if any, in the speed of the belt to obtain a previously set target speed, and a device which controls the belt speed with the thus-acquired speed adjustment until the next mark. If there is an unclear mark, control based on the previous mark is maintained to keep the speed constant. Further, if there is a stain or scratch, the calculation and acquisition of the speed adjustment is not performed, and speed adjustment based on the previous mark is maintained to keep the speed constant.
The first four background image forming apparatuses are similar in that they attempt to prevent the color shift in an image by controlling the rotational speed of the drive device. However, in the above background image forming apparatuses, abnormal control data detected due to a noise factor such as vibration, for example, may be used in a feedback operation, adversely affecting control performance. A method for preventing such deterioration of the control performance is not proposed.
Meanwhile, the fifth background image forming apparatus performs adjustment of the transfer position of an image, which is different from the belt drive control.